1. Field of the Invention
The present invention relates to a light path bending zoom optical system, especially, a zoom optical system and an image pickup apparatus provided with the same in which a light path bending prism is arranged in order to realize thinning of depth directions of a digital camera, a personal digital assistant (PDA), etc.
2. Description of the Related Art
In an image forming optical system which is built in and used for a miniature camera, a personal digital assistant, a cellular phone, etc., using an electronic image pickup element, such as CCD, C-MOS, etc., miniaturization, especially thinning of its shape has been strongly demanded. Furthermore, a zoom optical system with a high image forming performance which makes an angle of view variable by changing a focal length of image forming optical system is also demanded. To such a demand, many light path bending zoom optical systems in which a prism is arranged at an object side have been proposed and produced commercially. As for such light path bending zoom optical systems, in order to secure an angle of view at a wide angle side, to improve an image forming performance and to make a prism compact, in many cases, one or more negative lenses are arranged at an object side of the prism. On the other hand, as a zoom optical system without requiring such arrangement of the negative lens, for example, zoom optical systems have been proposed. Such systems have been disclosed in Publication of the Japanese unexamined patent application, Toku Kai No. 2003-43354, Publication of the Japanese unexamined patent application, Toku Kai. No. 2003-107356, Publication of the Japanese unexamined patent application, Toku Kai No. 2004-151552, and Publication of the Japanese unexamined patent application, Toku Kai No. 2004-264585.
In these conventional zoom optical systems, an optical system having a zoom ratio with about 3 times has been realized by constituting an entrance surface with concave shape, without arranging a lens element at the object side of a prism. In particular, in zoom optical systems shown in Publication of the Japanese unexamined patent application, Toku Kai No. 2003-43354 and [Publication of the Japanese unexamined patent application, Toku Kai No. 2003-107356, the exit surface of a prism are formed also to be concave surface. A full length of these zoom optical systems is about from 7 to 9 times of a diagonal length of an image pickup screen.